The SD card and multi-media card (MMC) standards use bi-directional bus lines. Specifically, the four data lines D0-D3 and the CMD lines are bi-directional, and the CLK line for a clock is uni-directional.
Conventionally, signal direction cannot be resolved by monitoring a simple condition. Instead, signal direction is determined by content of bus transactions; i.e., content of messages transferred over a bus.
The SD card and MMC standards also define voltage levels for signals. An SD card, for example, should operate in the 2.7V -3.6V range.
Some advanced silicon processes do not support voltages higher than 1.8V. For such processes, support of SD and MMC requires use of external level shifters, which boost voltages at a terminal. For a bi-directional bus connecting terminals A and B, a level shifter drives terminal A to 3V when terminal B is at 1.8V, for enabling a data signal to travel from A to B. Similarly, a level shifter drives terminal B to 3V when terminal A is at 1.8V, for enabling a data signal to travel from B to A. Thus level shifters require knowledge of signal direction in order to operate properly.
Conventional implementations of level shifting include an additional pin for each bus signal, to determine signal direction. Such an implementation is present in the Level Translator, Model SN74AVCA406 SMC/xD, manufactured and distributed by Texas Instruments, Inc. of Dallas, Tex. Integrated circuits that interface with such level shifters must support directional signals, in addition to the standard SD and MMC signals.
Support of directional signals causes large overhead and cost, for both the level shifter and the integrated circuit that interfaces with it. This is one of the drawbacks of bi-directional data buses.
Devices that require bridges between SD devices, such as a bridge between an SD host and an SD slave, also encounter the problem of determining signal direction. Moreover, often the SD signals being bridged to not have directional signals associated therewith, and thus their direction is unknown.
It would thus be of advantage to have circuitry and logic for determining signal direction in a bi-directional SD or MMC bus, without requiring external direction signals and without requiring decoding of exact content of bus transactions.